Lowest required signal-to-noise values, S/N₀, to reliably determine [Fe/H]₀, [Mg/H]₀ and [Ca/H]₀ abundances, evaluated at log g = 1.5 and log g = 4.5 and 4750 < T_eff < 6500 K (darker colors correspond to lower temperatures). The increase in T_eff complicates the [Fe/H] determination, mainly for Fe and Mg, and a higher S/N is necessary. The sensitivity of the Ca II HK resonance lines enables determining [Ca/H] at very low abundances.